I am repurposing a sea container (intermodal container) to be used as a root cellar. Sea containers are stacked for transit and the structural integrity for stacking is based on the four castings (corners) of the containers.

Corner Castings

The container will be buried with earth 2 feet deep. Since the the roof is not designed to be load bearing, the center of the container will need structural support. Soil on average weighs 80lb/sq cu ft. Therefore, the joists will need to support 160 lb/sqcu ft.

Height of joists and supports are not a factor. How do I determine the proper number and spacing for the joists?

  • 1
    Consider some central posts - if you are not driving a forklift into your root cellar, they are not going to be very much in the way, and can even be useful for organizing storage, while reducing the span for that heavy loading. Joist spacing depends on the unknown factor of "what loads can the roof material support?"
    – Ecnerwal
    Commented Dec 14, 2021 at 13:52
  • 2
    Probably too late, but if you haven't bought the container yet, you might find a large culvert (that you'd have to construct end walls for, but it would self-support soil loads without any additional structure) a more practical starting point for a root cellar. Or a Quonset hut for the flatter floor (ability to be buried was one factor in their favor for WWII.)
    – Ecnerwal
    Commented Dec 14, 2021 at 14:02
  • 2
    A cubic foot of soil weighs between 74 and 110 pounds, depending on the type of soil and how moist it is.. You should factor the worse-case scenario in your calculations. Also, once you do find some span charts make sure you're reading the dead load and not live load. Although live load will be important if you plan to drive over the shipping container. Don't forget to rubber-seal the entire outside of it because rusted sheet metal won't care how many supports you have.
    – MonkeyZeus
    Commented Dec 14, 2021 at 14:12
  • 1
    Here you go, awc.org/pdf/codes-standards/publications/span-tables/…
    – MonkeyZeus
    Commented Dec 14, 2021 at 14:16
  • 1
    Remember that whilst it is supposed to take load on corners, it's supposed to take hundreds of tons at the corner posts... You're adding tens of tons.
    – vidarlo
    Commented Dec 14, 2021 at 18:17

1 Answer 1


First, you need to determine the correct design loads -

  • Soil weight, ws = 120 pcf*2' = 240 psf

  • Live load, wl = 250 psf (allow heavy truck traffic) or 150 psf (mainly foot traffic)

  • Steel beam and panel weight, D, can be ignored at preliminary sizing, but need to be added to the design load for final design check.

Second, assume a beam spacing, let's say b = 5' o.c. (You shall make sure the roof panel is adequate in supporting the design load over this length)

Third, total design load on one beam, W = (ws+wL)*b + D, in plf

Fourth, assume simply supported beam, calculate the maximum moment, and check stress limit:

  • M = (W*L^2)/8, L is span length in ft

  • Bending stress, fb = M/Sx <= 0.6 fy ---> From here, you can figure out the required "section modulus, Sx" of the beam (in^3), and obtain the "moment of inertia, I (in^4)".

Finally, check the deflection of the selected beam:

  • Deflection = (5WL^4)/(384EI) <= L/240, or L/360 (preferred).

Iterate the process until a suitable/desirable beam size is selected.

Note: For steel, E = 29x10^6 psi; fy = 36 ksi or 50 ksi.

Comment: Figuring out beam size is only the first step in the entire design process. The next will be figuring out the beam end column size and then the footing size. Owing to all considerations, this task (design) belongs to a professional engineer rather than carried out by a DIYer.

  • This is not an answer. All you have done is cite some formulas without doing ANY calculations. Do you expect the op to do the calculations? Do you expect the op to know what “S” is or “M" or “I” is ? This is not an answer.
    – Lee Sam
    Commented Dec 14, 2021 at 17:33
  • @LeeSam I don't expect the OP to fully understand, so the answer is in the "comment",. But in case he is at the caliber as you, I put down the formulas and the respective limits for him to consider to size the beam as asked.
    – r13
    Commented Dec 14, 2021 at 17:47
  • @LeeSam But looks like I over-estimate your caliber in reading if you didn't notice all terms have been defined/addressed in the writing.
    – r13
    Commented Dec 14, 2021 at 17:51
  • @r13 Thanks for the detailed response. There are many opportunities for a responsible, intelligent and competent homeowner to find appropriate resources for a home improvement project. I chose to start at stack exchange. I’ve completed a plumbing project compliant with the IPC in this manner. Example: there are calculator’s that determine appropriate beam size: forestryforum.com/members/donp/beamclc06b.htm Commented Dec 14, 2021 at 19:27
  • I've never questioned the questioner's capability or intelligence in my responses, but always honestly express my views and concerns especially when personal safety is at stake. At this age, computer software and web advisors/articles often overstretched our capability without bound. Beware and good luck.
    – r13
    Commented Dec 14, 2021 at 19:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.